isolate GI and shadow on matte surface for re-build beauty in composite

[jasonhuang1115]

Hi,

I am doing a simple test where two spheres serving as foreground characters to be integrated into live-action plate. The spheres cast shadows onto and catch reflection from a ground plane set as matte surface. The material of the ground plane is assigned with a VRay material with some glossy reflection and projected plate image in the diffuse. I tried to break the rendering into two layers: 1. hero foreground character and 2. all the effects affecting the ground plane/matte surface. I then wanted to comp them together to have a 1-to-1 match to the beauty/ground truth where the two spheres integrated into the live-action plate.

Render settings:
beauty/ground truth render layer:
Background texture: Override Environment checked with plate projected with VRayPlaceEnvTex Mapping type set to Screen.
Vray object properties for spheres: default
Vray object properties for ground plane:
Matte Surface: on
Shadow: On

character render layer:
Background texture: Override Environment unchecked and is set to black.
Vray object properties for spheres: default
Vray object properties for ground plane:
Matte Surface: on
Alpha contribution: -1
Shadow: off

ground render layer:
Background texture: Override Environment unchecked and is set to black.
Vray object properties for spheres:
Primary Vis: off

Vray object properties for ground plane:
Matte Surface: on
Alpha contribution: -1
Shadow: on
Affect alpha: on

Both primary and secondary GI engines are set to Brute Force when GI is on and no GI Caustics.

The composite result and the beauty match 1 to 1 in terms of the character/spheres, but there are subtle differences on the ground in the shadows and GI affected area, if GI is on.
I wonder how can I isolate perfectly and shadow and received GI for a matte surface so they can be used to re-build the beauty.

composite result
beauty
difference

Vray 2.20.01 + Maya 2012

[jasonhuang1115]

the best way to set up shadow pass for integrating CG into live-action?

I have read through many related threads in this forum and the only approach I've tested so far that will give me a good 1-to-1 match between composite result and beauty is by following the tutorial posted here:
http://www.spot3d.com/vray/help/200R...rials_anim.htm

However, I am wondering if it is possible in current VRay, when having matte surfaces in a scene, by render the 3 images below to composite a 1-to-1 match of composite result and the beauty:
1. a character only render (where plate and matte surfaces appear black in the render)
2. a ground render that has correct GI, character reflection, and character shadows in separate render elements (background plate not appeared in this render)
3. a clean background plate

If it is not doable, what's the best way to set up the ground render that will contain close enough GI, character reflection, and character shadow information for compositing and live-action integration?

Thanks!